The Battle for Wesnoth is a Free, turn-based tactical strategy game with a high fantasy theme, featuring both single-player, and online/hotseat multiplayer combat. Fight a desperate battle to reclaim the throne of Wesnoth, or take hand in any number of other adventures. Easily modifiable. You can even download other users' add-ons from the main menu.
battle for wesnoth 1.8.6 windows installer full version
wesnoth 1.12.0 (stable): full version
wesnoth 1.12.0 linux (openpandora) full version
wesnoth 1.12.0 os x (10.5+) full version